Advanced Materials Research Vol. 1136

Abstract: Surface polishing is an important technique in the precision/ultra-precision process, especially in the optical industry. This paper introduces a new polishing method named Flexible Polishing by Compound Diamond Powder (FPCDP), in which the polishing abrasive grains are made of a kind of compound particles with flexible base and abrasive diamond powder. In FPCDP process, the compound abrasive grains are jetted onto the workpiece surface with an accelerated speed at an incident angle, and the micro irregularities on the workpiece surface are removed during the scratching, rolling and ploughing by the compound abrasive grains. The mechanism of polishing process is analyzed and the machining condition is optimized by experiments, which shows that FPCDP is an efficient method to improve the glass surface roughness.

Abstract: Shear-thickening polishing (STP) technology was used on ultraprecision machining of Si₃N₄ ceramics. The STP slurry with diamond abrasives was prepared for STP process and its rheological property was studied. The polishing performance of Si₃N₄ ceramics with STP was analyzed. Results show that STP slurry with diamond abrasives exhibits non-Newtonian power-law fluid characteristics with shear-thickening effect. As using STP slurry with abrasive particle size of 0.2 μm, the material removal rate changed from 4.22 to 4.05 μm/h after 60 mins ́ polishing; and decreased from 3.88 to 3.75 μm/h after 120 mins ́ polishing. The average surface roughness reduced from R_a 107.2 to R_a 6.5 nm after 120 mins ́ polishing.

Abstract: Ultra-precision grinding is widely used in machining of the hard and brittle materials due to its high surface accuracy and machining efficiency. However, grinding inevitably brings about surface and subsurface damage that needs to be removed by the polishing processes. This study investigated the surface and subsurface integrity of glass-ceramics induced by ultra-precision grinding. The characteristics of surface roughness, surface topography and subsurface damage depth of ground glass-ceramics with diamond grinding wheels with different grain sizes were presented and compared. Discussion was also provided to explore corresponding reasons of surface and subsurface integrity induced by diamond grinding wheels with different grain sizes.

Abstract: Large-aperture lens used for single-lens reflex cameras is generally produced by both ultra-precision grinding and polishing because a mass-produced lens by molding process can hardly achieve the required form accuracy and surface quality. Recently, grinding process of an optical glass should make surface quality as high as possible in order to reduce removal amount in polishing process. Accordingly, evaluation of subsurface damage induced under various grinding conditions has been required because subsurface damage leads to crack initiation or deterioration of the performance. However, evaluation method of subsurface damage on the spherical glass lens after grinding has not yet been established.In this study, the quantitative evaluation method by applying wet-etching of hydrofluoric acid (HF) is proposed, then its validity is verified. From the experimental results, we clarify to evaluate the subsurface damage by examining the surface transition according to etching time.

Abstract: Residual stresses in fused silica induced by two machining methods, diamond blade saw cutting and GC grinding wheel grinding, are studied in this study. Photoelastic method would be applied as measuring tool analyzing the gradient of residual stresses caused by different machining condition. This sentence is strange with analyzing the chips of grinding and observation of surface defects, it is assumed that the stresses gradient differs probably cause by the different material removal mechanism.

Abstract: Silicon nitride ceramic balls are successfully applied in advanced bearings. In order to improve the performance and precision for ball bearings, the research about the material properties of Si3N4 ball is shown in this paper. For the Si3N4 ball, the material properties can be described by the following factors: density, hardness, apparent porosity, crushing load and fracture toughness. In this paper, Si3N4 balls from three different manufacturers are tested to reveal the inherent correlations of the material properties, and the correlation coefficients are calculated. The results indicate that with the increasing of ball diameter, its crushing load increases correspondingly. The higher hardness contributes to the lower crushing load of the Si3N4 ball. Apparent porosity has a greater influence on the crushing load than hardness and the density. Hardness has the closest relevance with fracture toughness among the material characteristics, with the increasing of the hardness, the fracture toughness of Si3N4 ball becomes lower.

Abstract: This paper reports the stress distribution inside the workpiece under ultrasonic vibration cutting (UVC) condition. Many researchers have reported the improvements of tool wear, burr generation and surface integrity by reduction of time-averaged cutting force under UVC condition. However general dynamometers have an insufficient frequency band to observe the ultrasonically varying processing phenomena induced by UVC. In this paper, stress distribution inside the workpiece was observed by combining the pulse laser as light source synchronized with ultrasonically vibrating cutting tool and the photoelastic method. The one shot of pulse laser with pulse width of 15nsec visualizes an instantaneous stress distribution. Sweeping the phase of emission against to ultrasonic oscillation, 360 frames for 35.7μs, one period of ultrasonic oscillation, are captured. Because UVC induced an intermittent cutting condition, the stress distribution changed periodically and disappeared when the tool leaved from the workpiece. The ideal chip-generating period is calculated by relative motion between tool and work. We found that the actual chip-generating period was extremely longer than ideal period.

Abstract: Monolayer brazed diamond wheel were used to grind AlSiC composite with different volume fraction of SiC. The effects of diamond mesh size of the grinding wheels and the volume fraction of SiC in AlSiC composite on the surface integrity of ground AlSiC composite were investigated. It is found that the surface roughness of ground AlSiC composite with 20% SiC is increased with the reduction of diamond grit’s size because of the clogging of diamond wheel. With the increasing of the volume fraction of SiC, there are more fractured and cracked SiC particles on the ground surface of AlSiC composite. On the other hand, the finer diamond grits (70/80) induce the finer fractured SiC particles on the ground surface of AlSiC with 50% and 70% SiC.

Abstract: To grind fused silica in ductile mode, it was proposed to repair surface and subsurface micro cracks of fused silica by CO₂ laser irradiation. However, excessive residual stress remains on the surface because the melt fused silica on the surface quenches in air. It causes the critical depth of cut for ductile grinding fused silica to be smaller than 0.2μm. To investigate the distribution of the residual stress and look for an optimal manner of irradiation to control residual tensile stress, a numerical model of was built for simulating the dynamic behavior of fused silica when irradiated by CO₂ laser. Laser energy absorption, heat transmission, viscoelastic behavior of fused silica and thermally induced stress were considered in the numerical simulation. The results show how the residual stress is formed and distributed. We found that an appropriate control of the temperature field as a function of time and position in the laser process is the key to reduce the residual stress. Therefore, three kinds of processes were proposed to reduce residual tensile stress on the surface of fused silica introduced by laser irradiation. The residual stress distributions of these three processes were compared by numerical analysis to decide a better method of laser irradiation.

Abstract: The compressor blade is the key components of the aero-engine to seriously impact the air dynamic performance. However, components with complex designs specifications might pose manufacturing challenges especially when finishing processes are needed to enable their compliance with tight industrial standards for workpiece surface integrity. Information on polishing processes for such sensitive industrial applications is scarce. The paper reports on the influence of polishing methods on the surface integrity of compressor blade obtained after different polishing methods. The research focuses on identifying an “optimised” polishing method for different area that will enable finishing the compressor blade. Two (belt; bob) polishing methods have been tested to address the overall finishing of compressor blade. Although significant differences in tool life performance exist between belt and bob polishing methods, both are capable to meet the requirements of minimum workpiece surface coverage if “optimised” operating parameters are employed. This is proved that belt and bob polishing methods can be employed to enable automated overall finishing of compressor blade.